No, Metroid belongs to the harder games. Although once you know your way around, it is a lot easier. Not just in the sense that all games become more manageable once you know what you have to do, it has a bigger impact than normal on Metroid.

First time is very challenging, at least until you stumble across the screw attack. Being invincible and dealing damage in mid-air makes wandering through the countless corridors quite comfortable. That doesn't help with the bosses and metroids, but it's still the best power-up in the game.

Mega Man 3 has medium difficulty, although the middle of the game presents a notable spike in difficulty. Like always, once you've figured out which weapons are effective against bosses, you have a lot less to fear.

Link's Awakening is medium too, although the first boss is quite tough.

The first Mega Man game is hard. Zelda 2 is hard.

But you are right, Ghost'n'Goblins is in a different league.

The ones I play are mostly medium, occasionally easy to medium, rarely medium to hard or outright hard. It would be a mistake to label new entries in long-running series as easy, because a lot of the perceived easiness stems from experience with the gameplay formula. If you watch an inexperienced player, you are reminded of your own struggles when you first played video games.

The brutally hard games have died out, but that's a good thing. Their difficulty was rooted in poor game design, so nothing of value was lost.